Assessing the accuracy of road-side systems

ABSTRACT

A method of assessing the accuracy of a roadside traffic monitoring system (TMS) arranged to measure the speed and/or other parameters of vehicles passing a predetermined measurement point adjacent the TMS is disclosed. The method includes measuring the speed of each vehicle passing the measurement point using the TMS, measuring the moment in time that each vehicle passes the measurement point using the TMS, and recording the measured speeds and times as data pairs. The method further includes driving an Instrumented Probe Vehicle (IPV) at the measurement point, the IPV having an onboard system for measuring speed, determining the location of the IPV, determining a calibration time at which the determined location of the IPV corresponds to the location of the measurement point, and measuring a calibration speed of the IPV using the onboard system when the determined location of the IPV corresponds to the location of the measurement point.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a National Phase Application of InternationalApplication No. PCT/GB02/05384, filed Nov. 28, 2002, which claimspriority from Great Britain Patent Application No. 0201168.2, filed Jan.18, 2002. The present application claims priority from bothapplications.

FIELD

The present invention generally relates to the assessment of theaccuracy of road-side systems, and more particularly but not exclusivelyto the assessment of road-side Traffic Monitoring Stations (TMS).

BACKGROUND

A highway operator often wishes to gather information about vehiclesusing the highway. The speeds and journey times of vehicles areparticularly of interest. For example, the operator of a motorway fromLondon to Bristol may wish to know the speed of individual vehicles atone or a number of locations. The instantaneous speeds of vehicles atpredefined locations are known as “spot speeds”. The operator may alsowish to know the average travel time between London and Bristol, forexample, or for sections of the route. This travel time can be estimatedfrom the spot speeds measured at the measurement points. The methods tointegrate the journey time from the spot speeds are well known and willnot be described herein.

There are two roadside systems in general use for measuring the speed ofvehicles at a particular location: One of these uses two sensors a fixeddistance apart. Sensors can for example take the form of light beamsarranged to be broken by passing vehicles, or electromagnetic coils orpressure sensors buried in the roadway. The time taken for a vehicle topass from one sensor to the other is measured, and the speed of thevehicle can be calculated from this “time of flight”. Roadside systemsthat use such a system have the problem that over time they may driftout of calibration.

Another roadside system in general use takes advantage of the Dopplereffect. A radar source is directed towards oncoming traffic, and radiowaves reflected back towards the source from the moving traffic aredetected. The speed of a vehicle travelling towards such a radar sourcecan be calculated from the change in frequency of the radio wavesreflected from that vehicle. Such systems are unlikely to drift out ofcalibration over time. However, systems with Doppler radar are subjectto installation and orientation errors that introduce the “cosineeffect” whereby all speeds of vehicles are under-read by a certainproportion, determined by the angle of the radar beam relative to thevehicle direction.

Before the results of spot speed measurement can be used for analysis ofthe traffic stream, the accuracy of each measurement station needs to beassessed. Final results are not useful unless a confidence limit can bedetermined for the spot speed of all vehicles at each site and theaverage speeds for all vehicles at a selection of sites constituting ajourney. Furthermore, measurement stations need to be assessed foraccuracy at regular time intervals following their initial installation,to confirm that they have not drifted away from calibration. Typically,measurement stations need to be assessed approximately every threemonths.

The equipment and method for assessing measurement stations needs to besuitable for fast and efficient verification of speed monitoringequipment. This means that the system must be portable and suitable forquick deployment or assessment.

At present, systems for speed measurement assessment include thefollowing methods:

-   -   Radar (Doppler) or LIDAR (Laser Diode Ranging).    -   Two light beams horizontally or vertically across the        carriageway.    -   Two pressure sensors on the road surface.

Radar devices use the Doppler effect as described above. When portabledevices are used, the radio source and receiver are located in a handheld device (a “speed gun”). Such devices are very accurate when used insuitable conditions, but can still give rise to a number of drawbacks.Firstly, when a motorist sees a speed gun in use, they will often applythe brakes, or at least take their foot off the accelerator. This meansthat the vehicle will be slowing as it passes the sensor and this willintroduce a measurement error. Furthermore, the method is verylabour-intensive and difficult to use in heavy traffic. There are errorsintroduced by the “cosine” effect, the effect of the angle between thegun beam and the vehicle direction.

Two horizontal light beams or pressure sensors on the road surface maybe used successfully in low volume single lane carriageways. However,many modern roads are dense dual carriageways, and these methods areimpractical in practice. Installing sensors on the road is hazardous andcan easily lead to an accident.

Thus the present methods for assessing the accuracy of road-sidemeasurement are relatively inefficient, inaccurate and can be unsafe touse.

SUMMARY

In accordance with a first aspect of the present invention there isprovided apparatus for assessing the accuracy of a roadside trafficmonitoring station (MS) having data measurement means for measuring aparameter of vehicles passing a predetermined measurement point and themoment in time at which each vehicle passes the measurement point, theapparatus comprising:

-   -   data recording means for recording the parameter of each vehicle        and moment in time that vehicle passes the measurement point as        measured by the data measurement means;    -   an Instrumented Probe Vehicle (IPV) having an onboard measuring        system for measuring the parameter of the IPV;    -   locating means associated with the IPV for determining the        location of the IPV independently of the TMS;    -   timing means for determining the moment in time at which the        location of the IPV as measured by the locating means        corresponds to the location of the measurement point; and    -   data processing means for identifying the parameter of the IPV        as measured by the parameter measurement means from the moment        in time at which the IPV passes the measurement point.

This means that the TMS can be assessed for accuracy by comparing theparameter of the IPV as measured by the TMS with the parameter of theIPV as known beforehand or determined by the IPV onboard measurementsystem. The measured parameter may be one or more of speed, vehiclelength, width, height, gross weight, axle weight, and wheelconfiguration. This allows the accuracy of the TMS to be assessed easilyby driving an IPV connected to suitable locating means past the TMS,requiring very little operator skill. Since no road-side operations arerequired, the safety of personnel carrying out the assessment isincreased. There is little or no disruption or disturbance of thevehicle stream.

The locating and/or timing means may conveniently include a GlobalPositioning System (GPS).

The apparatus preferably comprises a communication system so that datacan pass from the IPV to the data processing means, the data processingmeans being arranged to receive data from the IPV corresponding to thetime at which the location of the IPV corresponds to the location of themeasurement point, and to identify a data pair in the data recordingmeans corresponding to that time, said data pair including the parameterof the IPV as measured by the TMS. The IPV may include means forrecording the parameter at the moment in time at which the IPV islocated at the measurement point and transmitting this information tothe data processing system.

The communication means preferably includes a mobile phone system,enabling the IPV to send data to the data processing means as a ShortMessage Service (SMS) message.

The parameter may be speed, and the IPV preferably includes onboardspeed measurement and recording means for measuring and recording thespeed at the moment in time at which the IPV is located at themeasurement point.

In accordance with a second aspect of the present invention there isprovided apparatus for assessing the accuracy of a roadside trafficmonitoring station (TMS) having data determination means for determiningthe measured speed of vehicles passing a measurement location and themeasured moment in time at which each vehicle passes the measurementlocation, the apparatus comprising:

-   -   an Instrumented Probe Vehicle (IPV) having on board speed        measuring means for measuring the speed of the IPV;    -   locating means for determining the location of the IPV        independently of the TMS; and    -   recording means for recording a calibration speed of the IPV as        measured by the on board speed measuring means when the IPV is        located at the measurement location as determined by the        locating means, together with a calibration time at which the        IPV is located at the measurement location.

The apparatus preferably comprises communication means for communicatingthe calibration speed and calibration time of the IPV to the TMS, andmay include data processing means for comparing the calibration timereceived from the IPV with the measured times of vehicles passing themeasurement location so as identify the measured speed of the IPV andcompare it with the calibration speed.

Similar apparatus may be used to assess the accuracy of a TMS arrangedto measure the weight of passing vehicles instead of or in addition totheir speed. The IPV may include a dynamic weight determination meansenabling a calibration weight to be determined when the IPV is locatedat the measurement point.

In accordance with a third aspect of the present invention there isprovided a method of assessing the accuracy of a roadside trafficmonitoring station (TMS) arranged to measure a parameter of vehiclespassing a predetermined measurement point, the method comprising:

-   -   measuring the parameter of each vehicle passing the measurement        point using the TMS;    -   measuring the moment in time that each vehicle passes the        measurement point using the TMS;    -   recording the measured parameters and times as data pairs;    -   driving an Instrumented Probe Vehicle (IPV) past the measurement        point, the IPV arranged so that the parameter thereof is known        or is measurable using an onboard measuring system;    -   determining the location of the IPV using location means;    -   determining the time at which the determined location of the IPV        corresponds to the location of the measurement point;    -   sending the determined time, and the parameter of the IPV as        known or measured using the on board system, to the TMS;    -   identifying the recorded data pair corresponding to the        determined time;    -   identifying the parameter of the IPV, as measured by the TMS,        from said identified data pair; and    -   comparing the parameter of the IPV, as measured by the TMS, with        the parameter of the IPV, as known or measured by the onboard        measuring system.

The parameter may be vehicle speed, and the speed of the IPV ispreferably measured using an onboard system and recorded at the momentthe location of the IPV, as determined by the location means,corresponds to the measurement point.

The method preferably further comprises driving the IPV past a pluralityof measurement points and comparing the parameter of the IPV, as knownor measured by an onboard measuring system, with the parameter of theIPV as measured at each measurement point.

In accordance with a fourth aspect of the present invention there isprovided a method of assessing the accuracy of a roadside trafficmonitoring station (TMS) arranged to measure the speed of vehiclespassing a predetermined measurement point, the method comprising:

-   -   measuring the speed of each vehicle passing the measurement        point using the TMS;    -   measuring the moment in time that each vehicle passes the        measurement point using the TMS;    -   recording the measured speeds and times as data pairs;    -   driving an Instrumented Probe Vehicle (IPV) past the measurement        point, the IPV having an onboard system for measuring speed;    -   determining the location of the IPV using location means;    -   determining a calibration time at which the determined location        of the IPV corresponds to the location of the measurement point;    -   measuring a calibration speed of the IPV using the onboard        system when the determined location of the IPV corresponds to        the location of the measurement point;    -   sending the calibration time and calibration speed to the TMS;    -   identifying the recorded data pair corresponding to the        calibration time;    -   identifying the speed of the IPV, as measured by the TMS, from        said identified data pair; and    -   comparing the speed of the IPV, as measured by the TMS, with the        calibration speed of the IPV, as measured by the onboard        measuring system.

A similar method may be used to assess the accuracy of a“weigh-in-motion” system, although in such a case the IPV may not needonboard weight measuring means. The weight of the IPV can be determinedbefore it is driven past the measurement point. However, for moreaccurate assessment the IPV may be fitted with an onboard dynamicweighing system (well known in prior art) which reports theinstantaneous wheel loads continuously. As with the speed measurement,these wheel loads may be captured at the appropriate moment as the IPVpasses the measurement point and sent to the TMS with the calibrationtime at which the IPV passes the measurement point. In that way thedynamic effect of undulations in the road surface which lead to vehiclebounce and thus a dynamic element in the vehicle load on the road can beisolated.

The IPV may be a maintenance or operations vehicle. Such vehicles may beused on the section of highway on which the traffic monitoring stationsare located to monitor the condition of the highway. This has theadvantage of reducing costs, as roadside systems can be calibrated usingvehicles which would be passing the stations in any case.

In accordance with a fifth aspect of the present invention there isprovided a computer storage medium having stored thereon a programarranged when executed to enable a processor to:

-   -   receive data containing matched record pairs corresponding to a        parameter of a vehicle passing a TMS together with the time at        which said vehicle passes said TMS, each as measured by the TMS;    -   receive data from an IPV containing information about the        parameter of the IPV, together with the time at which the IPV        passes the TMS, each as determined by an onboard measurement        system of the IPV;    -   identify which matched record pair corresponds to the passage of        the IPV by comparing the time at which the IPV passes the TMS as        determined by the on board measurement system with the time at        which vehicles pass the TMS as measured by the TMS; and    -   determine the parameter of the IPV as measured by the TMS from        the identified matched record pair.

The program is preferably further arranged to enable the processor tocompare the parameter of the IPV as measured by the TMS with theparameter of the IPV as measured by the onboard measurement system so asto determined the measurement accuracy of the TMS.

BRIEF DESCRIPTION OF THE DRAWINGS

Some preferred embodiments of the invention will now be described by wayof example only and with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic diagram showing the general layout of a section ofhighway;

FIG. 2 shows the components of a traffic monitoring station (TMS);

FIG. 3 is a perspective view of part of a TMS; and

FIG. 4 shows the components used to match Instrumented Probe Vehicle(IPV) measurements with TMS measurements.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram showing the general layout of a section ofhighway 100 having four traffic measurement stations 101, 102, 103 and104. In this example, the stations are installed at 500 metre intervals.Each measurement station is arranged to measure the speed of vehiclespassing the measurement station and transmit this information to acentral location, or instation, 111. The time of journey for vehiclestravelling along the highway can be estimated from the speeds ofvehicles passing the four measuring stations.

FIG. 2 shows the components of an individual measurement station 101,arranged to measure the speeds of vehicles in both carriageways of amotorway 122, i.e. six lanes of traffic 123, 124, 125, 126, 127, 128.The measurement station comprises wire loops 129 located under thesurface of the roadway 122, two loops being located under each lane oftraffic a fixed distance D apart. The following discussion will considerthe two loops 130, 131 located in the first lane of traffic 123, but itwill be appreciated that the same considerations will apply for all ofthe other lanes.

Each loop 130, 131 is about two metres square and consists of 3 turns ofwire. As a vehicle passes over the loop it causes a change in theinductance of the loop, and this can be detected by “loop detectors”(not shown) attached to the loop. The loop detectors are connected to ameasurement and control unit 121 which includes processing means foranalysing information passed to the measurement and control unit by theloop detectors. The loop detectors can be arranged to provide ananalogue representation of the passing of each vehicle, or alternativelycan be set to be switched “on” or “off” by the passage of a vehicle.Every time a vehicle is detected by a loop detector this information ispassed to the measurement and control unit 121. The speed of a vehiclepassing the loops 130, 131 is determined by the measurement and controlunit 121 from the time it takes between detection by the two loops. Thisgives the time for the vehicle to travel the fixed distance D, and thusits speed over that distance.

The point in the lane half way between the two loops is known as the“measurement location” 132 for that measurement station. Each lane willhave its own measurement location, so that the measurement station 101has associated with it six measurement locations. FIG. 3 shows aperspective view of one carriageway of the motorway 122 at themeasurement station 101, at the moment that a car 133 crosses themeasurement point 132 for the first lane 122. The measurement andcontrol unit 121 passes information about the speed of each vehiclewhich passes each measurement point, and the moment in time when thistakes place, back to the central instation 111 (see FIG. 1) which isarranged to correlate information from all of the measurement stations.In practice, loop sensors and other sensors and/or detectors may have anumber of further distinct detection points at which a passing vehiclecan be detected and its speed measured whilst the vehicle is in asensing area or at a sensing point. The entire system of detectorsinvolved in the roadside processing is known as a traffic monitoringsystem (TMS).

FIG. 4 shows a general overview of the system used to check the accuracyof the TMS. An instrumented probe vehicle (IPV) 141 is fitted with acustom speed monitoring and control system that enables its speed to bemeasured to ±0.3 mph (at a 95% confidence level). The IPV is also fittedwith a transmitter and receiver device 145 which allows it to connect toan accurate locating system such as the Global Positioning System (GPS)146. The IPV 141 is also fitted with an on-board computer, into which isprogrammed the exact position of the measurement point of each lane ofeach TMS.

The IPV is driven past each of the measurement stations. As the vehicleis in motion, it continually calculates the position of the nearest TMS101 and lane locations. As the IPV approaches a TMS location 101, onceit is within a certain minimum distance, it monitors its position and atthe very point where the change in the distance to the nearestmeasurement point 132 is zero, it calculates it own precise speed anddirection. This value is transferred to a temporary storage positiontogether with the exact time (HH:MM:SS.TTTT) also derived from the GPSsignal and an indication of the measurement point 132 passed, and thelocation of the TMS 101.

This data is then assembled as a Short Message Service (SMS) datamessage and sent via a transmitter 144 to the address of the centralinstation 111. The instation 111 interrogates the control andmeasurement unit 121 of the specified TMS 101 and from the time andlocation information the IPV is identified from the record of thepassage of individual vehicles held at the TMS 101.

In other words, as the IPV 141 is driven past the measurement station101, its speed is measured both by the onboard system of the IPV itselfand by the measurement station 101. By using GPS to determine the exacttime at which the IPV passes the station, the two records can bematched. In practice, the process gathers matched records over one ormore days of operations, and an average result is obtained. Typically,for statistical significance, when a normal distribution is known, about6 minimum samples are required.

This accuracy assessment is conducted at periodic intervals, typicallyevery three months (i.e. four times per year) for all of the TMSmeasurement points maintained by a highway operator. Starting at thefirst measurement station, an IPV passes all the TMS sites on a circuit.The driver proceeds with the traffic stream, but not above the speedlimit. For each site, the speed as measured by the IPV is sent to thecentral instation 111 and matched with a corresponding record from theTMS, as described above. The round trip might take from 20 minutes to afew hours including rest breaks. In the survey period, the vehicleshould make a minimum of six passes through each lane measurement point.The IPV thus obtains a minimum of 6 samples for each lane at each site.The IPV and TMS records are then analysed at the instation 111 for meanerror and standard deviation.

The following example illustrates analysis applied to data derived fromsix vehicle passes at a single measurement point on a test track.

IPV Speed TMS Speed Absolute Vehicle Report Report Error Error pass(km/hr) (km/hr) (km/hr) (%) 1 147.2 147.5 +0.3 0.20% 2 95.7 95.5 −0.2−0.21% 3 101.0 101.5 +0.5 0.50% 4 97.3 97.5 +0.2 0.21% 5 147.9 147.5−0.4 −0.27% 6 95.5 95.5 +0.0 0.00% Average Mean 0.13 0.20% SD 0.39 0.60%

The TMS output in this case is shown to 0.1 km/hr during verification.This enables minimum error due to rounding when performing the errorsurvey.

The statistics for the percentage error column are calculated: the meanspeed error for the survey was 0.20% while the standard deviation (SD)was 0.60%.

From this the average error for all vehicles can be calculated usingStudent's t from the standard statistical tables for six samples:

${{CI}({Average})}_{95\%} = {{{\pm t_{95,n}} \times \frac{SD}{\sqrt{n}}} = {{2.57 \times \frac{0.60\%}{\sqrt{6}}} = {{\pm 0.63}\%}}}$

Thus the true mean speed for all vehicles will be between +0.20%–0.63%and +0.20%+0.63%, i.e. between −0.43% and +0.83%, of the mean speed,calculated from the equipment reports with a confidence level of 95%.Since these values are within ±1%, the station is verified to meet theperformance requirement.

To calculate the individual vehicle speed variation:CI(Individual)_(95%) =±t _(95,n) ×SD=2.57×0.602%=±1.57%

This means that individual speed reports will lie between +0.20%–1.57%and +0.20%+1.57%, i.e. −1.37% and +1.77% at a confidence level of 95%.Since these values are within ±2%, the station is verified to meet theperformance requirement.

Thus a reliable estimate of the performance of the TMS has been gainedgiving data on the measurement of individual vehicles, the way thisextrapolates to the mean speed of all vehicles, and systematic bias.This information has been gathered in a safer and more accurate methodand at lower cost than the existing methods.

The above description refers to the calibration of speed measurementsystems designed to be used in lanes of traffic. Sometimes such speedmeasurement systems also have measurement points in the “hard shoulder”of a motorway. For safety reasons, hard shoulder verification is notnormally performed. If hard shoulder verification is required, the hardshoulder passes should be at “hard shoulder speeds”, i.e. around halfnormal flow speed. The test would be abandoned if any abnormality intraffic flow is observed or if the hard shoulder is occupied within akilometre of the TMS. If the hard shoulder were being used as a runninglane at the time of verification, it would be verified at that time,although the closed lane would not be verified on that occasion.

The calibration systems and methods described above provide higherlevels of accuracy than were possible with previous apparatus andmethods. Since no road-side operations are required, the safety ofpersonnel carrying out the assessment is increased. There is little orno disruption or disturbance of the vehicle stream, and the performanceof the TMSs can be checked at any time. When a high density of stationsis involved, the time per test can be as little at 10–30 seconds withlittle or no operator experience. The audit records created areimpossible for the test operator to influence or corrupt in any way.

It may be possible to reduce costs even further if the IPV is a dualpurpose vehicle used normally for maintenance on the section of roadalong which the TMS devices are located. It is usual for maintenancevehicles to make three or four runs along the highway each day to checkfor debris and broken down vehicles, check the condition of signpostsand the surface of the road, etc. In such a case the necessarycomponents for recording and forwarding times and speeds can beinstalled in the maintenance vehicle and left running permanently,enabling almost continuous assessments of the performance of the TMSdevices.

It will be appreciated that the invention is not limited to theembodiment described above, and may also be used for calibration ofother equipment. For example, an IPV of known weight could be used toassess the accuracy of “weigh-in-motion” systems which determine theweight of vehicles crossing them. Such systems require frequentrecalibration to remain inside specified limits. Weight sensors for suchsystems are also sometimes attached to TMS devices. By determining theexact position of the IPV at all times using GPS or similar, and knowingthe exact position of the weight sensor, the exact moment in time atwhich it crosses (and is weighed by) a weigh-in-motion sensor point canbe determined. This information can be sent to the weigh-in-motionsystem together with the known weight of the IPV, enabling the recordcorresponding to the IPV to be extracted from the weigh-in-motion systemrecords and compared with this known weight. A similar system may beused to calibrate measurement systems for vehicle length, width, height,gross weight, axle weight, or wheel configuration, for example.

It will be appreciated that other departures from the embodimentsdescribed above may stall fall within the scope of the invention.

1. Apparatus for assessing the accuracy of a roadside traffic monitoringstation having data measurement means for measuring a parameter ofvehicles passing a predetermined measurement point and the moment intime at which each vehicle passes the measurement point, the apparatuscomprising: data recording means for recording the parameter of eachvehicle and moment in time that vehicle passes the measurement point asmeasured by the data measurement means; an Instrumented Probe Vehiclehaving an onboard measuring system for measuring the parameter of theInstrumented Probe Vehicle; locating means associated with theInstrumented Probe Vehicle for determining the location of the IPVindependently of the traffic monitoring station; timing means fordetermining the moment in time at which the location of the InstrumentedProbe Vehicle as measured by the locating means corresponds to thelocation of the measurement point; and data processing means foridentifying the parameter of the Instrumented Probe Vehicle as measuredby the data measurement means from the moment in time at which theInstrumented Probe Vehicle passes the measurement point.
 2. Apparatus asclaimed in claim 1, wherein the locating means includes a GlobalPositioning System.
 3. Apparatus as claimed in claim 1, wherein thetiming means includes a Global Positioning System.
 4. Apparatus asclaimed in claim 1 and comprising a communication system for passingdata from the Instrumented Probe Vehicle to the data processing means,so that the data processing means can receive data from the InstrumentedProbe Vehicle corresponding to the time at which the location of theInstrumented Probe Vehicle as measured by the locating means correspondsto the location of the measurement point, and can identify a data pairin the data recording means corresponding to that time, said data pairincluding the parameter of the Instrumented Probe Vehicle as measured bythe traffic monitoring station.
 5. Apparatus as claimed in claim 4,wherein the Instrumented Probe Vehicle includes means for recording theparameter as measured by the onboard measuring system at the moment intime at which the Instrumented Probe Vehicle is located at themeasurement point as determined by the locating means, and transmittingthis information to the data processing means.
 6. Apparatus as claimedin claim 5, wherein the communication system includes a mobile phonesystem.
 7. Apparatus as claimed in claim 6, wherein the InstrumentedProbe Vehicle is arranged to send data to the data processing means as aShort Message Service message.
 8. Apparatus as claimed in claim 1,wherein the parameter is speed and the Instrumented Probe Vehicleincludes onboard speed measurement and recording means for measuring andrecording the speed at the moment in time at which the InstrumentedProbe Vehicle is located at the measurement point.
 9. Apparatus asclaimed in claim 1, wherein the parameter is vehicle length, width,height, gross weight, axle weight, or wheel configuration.
 10. Apparatusfor assessing the accuracy of a roadside traffic monitoring stationhaving data determination means for determining the measured speed ofvehicles passing a measurement location and the measured moment in timeat which each vehicle passes the measurement location, the apparatuscomprising: an Instrumented Probe Vehicle having on board speedmeasuring means for measuring the speed of the Instrumented ProbeVehicle; locating means associated with the Instrumented Probe Vehiclefor determining the location of the Instrumented Probe Vehicleindependently of the traffic monitoring station; and recording means forrecording a calibration speed of the Instrumented Probe Vehicle asmeasured by the on board speed measuring means when the location of theInstrumented Probe Vehicle as determined by the locating meanscorresponds to the measurement location, together with a calibrationtime at which the Instrumented Probe Vehicle is located at themeasurement location as determined by the locating means.
 11. Apparatusas claimed in claim 10, further comprising communication means forcommunicating the calibration speed and calibration time of the IPV tothe traffic monitoring station.
 12. Apparatus as claimed in claim 11,further comprising data processing means for comparing the calibrationtime received from the Instrumented Probe Vehicle with the measuredtimes of vehicles passing the measurement location so as identify themeasured speed of the Instrumented Probe Vehicle and compare it with thecalibration speed.
 13. Apparatus for assessing the accuracy of aroadside traffic monitoring station having data determination means fordetermining the measured weight of vehicles passing a measurementlocation and the measured moment in time at which each vehicle passesthe measurement location, the apparatus comprising: an InstrumentedProbe Vehicle having a known weight; locating means associated with theInstrumented Probe Vehicle for determining the location of InstrumentedProbe Vehicle independently of the traffic monitoring station; andrecording means arranged to record a calibration time at which thelocation of the Instrumented Probe Vehicle as determined by the locatingmeans corresponds to the measurement location.
 14. Apparatus as claimedin claim 13, wherein the Instrumented Probe Vehicle is fitted with anonboard dynamic weighing system which reports instantaneous wheel loadsso as to determine the weight of the vehicle, and wherein the recordingmeans is arranged to record a calibration weight of the InstrumentedProbe Vehicle as determined by the on board weighing system at themoment in time the Instrumented Probe Vehicle is located at themeasurement location, together with the calibration time at which theInstrumented Probe Vehicle is located at the measurement location. 15.Apparatus as claimed in claim 13, wherein the Instrumented Probe Vehicleis a maintenance or operations vehicle.
 16. A method of assessing theaccuracy of a roadside traffic monitoring station arranged to measure aparameter of vehicles passing a predetermined measurement point, themethod comprising: measuring the parameter of each vehicle passing themeasurement point using the traffic monitoring station; measuring themoment in time that each vehicle passes the measurement point using thetraffic monitoring station; recording the measured parameters and timesas data pairs; driving an Instrumented Probe Vehicle past themeasurement point, the IPV arranged so that the parameter thereof isknown or is measurable using an onboard measuring system; determiningthe location of the Instrumented Probe Vehicle using location means;determining the time at which the determined location of theInstrumented Probe Vehicle corresponds to the location of themeasurement point; sending the determined time, and the parameter of theInstrumented Probe Vehicle as known or measured using the on boardsystem, to the traffic monitoring station; identifying the recorded datapair corresponding to the determined time; identifying the parameter ofthe Instrumented Probe Vehicle, as measured by the traffic monitoringstation, from said identified data pair; and comparing the parameter ofthe Instrumented Probe Vehicle, as measured by the traffic monitoringstation, with the parameter of the Instrumented Probe Vehicle, as knownor measured by the onboard measuring system.
 17. A method as claimed inclaim 16, wherein the location means includes a Global PositioningSystem.
 18. A method as claimed in claim 16, wherein the parameter isvehicle speed, and wherein the speed of the Instrumented Probe Vehicleis measured using an onboard system and recorded at the moment thelocation of the Instrumented Probe Vehicle, as determined by thelocation means, corresponds to the measurement point.
 19. A method asclaimed in claim 16, further comprising driving the Instrumented ProbeVehicle past a plurality of measurement points and comparing theparameter of the Instrumented Probe Vehicle, as known or measured by anonboard measuring system, at each measurement point, with the parameterof the Instrumented Probe Vehicle as measured using the trafficmonitoring station at that measurement point.
 20. A method of assessingthe accuracy of a roadside traffic monitoring station arranged tomeasure the speed of vehicles passing a predetermined measurement point,the method comprising: measuring the speed of each vehicle passing themeasurement point using the traffic monitoring station; measuring themoment in time that each vehicle passes the measurement point using thetraffic monitoring station; recording the measured speeds and times asdata pairs; driving an Instrumented Probe Vehicle past the measurementpoint, the IPV having an onboard system for measuring speed; determiningthe location of the Instrumented Probe Vehicle using location means;determining a calibration time at which the determined location of theInstrumented Probe Vehicle corresponds to the location of themeasurement point; measuring a calibration speed of the InstrumentedProbe Vehicle using the onboard system when the determined location ofthe Instrumented Probe Vehicle corresponds to the location of themeasurement point; sending the calibration time and calibration speed tothe traffic monitoring station; identifying the recorded data paircorresponding to the calibration time; identifying the speed of theInstrumented Probe Vehicle, as measured by the traffic monitoringstation, from said identified data pair; and comparing the speed of theInstrumented Probe Vehicle, as measured by the traffic monitoringstation, with the calibration speed of the Instrumented Probe Vehicle,as measured by the onboard measuring system.
 21. A method of assessingthe accuracy of a roadside traffic monitoring station arranged tomeasure the weight of vehicles passing a predetermined measurementpoint, the method comprising: measuring the weight of each vehiclepassing the measurement point using the traffic monitoring station;measuring the moment in time that each vehicle passes the measurementpoint using the traffic monitoring station; recording the measuredweights and times as data pairs; determining a calibration weight of anInstrumented Probe Vehicle using a weight determination means associatedwith the Instrumented Probe Vehicle; driving the Instrumented ProbeVehicle past the measurement point; determining the location of theInstrumented Probe Vehicle using location means; determining acalibration time at which the determined location of the InstrumentedProbe Vehicle corresponds to the location of the measurement point;sending the calibration time and calibration weight to the trafficmonitoring station; identifying the recorded data pair corresponding tothe calibration time; identifying the weight of the Instrumented ProbeVehicle, as measured by the traffic monitoring station, from saididentified data pair; and comparing the weight of the Instrumented ProbeVehicle, as measured by the traffic monitoring station, with thecalibration weight of the Instrumented Probe Vehicle, as determined bythe weight determination means.
 22. A method as claimed in claim 21,wherein the weight determination means is an onboard dynamic weighingsystem which reports instantaneous wheel loads so as to determine theweight of the vehicle, and wherein the calibration weight is determinedwhen the Instrumented Probe Vehicle is located at the measurement point.23. A method as claimed in of claim 16, wherein the Instrumented ProbeVehicle is a maintenance or operations vehicle.
 24. A computer storagemedium having stored thereon a program arranged when executed to enablea processor to: receive data containing matched record pairscorresponding to a parameter of a vehicle passing a traffic monitoringstation together with the time at which said vehicle passes said trafficmonitoring station, each as measured by the traffic monitoring station;receive data from an Instrumented Probe Vehicle containing informationabout the parameter of the Instrumented Probe Vehicle, together with thetime at which the Instrumented Probe Vehicle passes the trafficmonitoring station, each as determined by an onboard measurement systemof the Instrumented Probe Vehicle; identify which matched record paircorresponds to the passage of the Instrumented Probe Vehicle bycomparing the time at which the IPV passes the traffic monitoringstation as determined by the on board measurement system with the timeat which vehicles pass the traffic monitoring station as measured by thetraffic monitoring station; and determine the parameter of theInstrumented Probe Vehicle as measured by the traffic monitoring stationfrom the identified matched record pair.
 25. A computer storage mediumas claimed in claim 24, wherein the program is further arranged toenable the processor to compare the parameter of the Instrumented ProbeVehicle as measured by the traffic monitoring station with the parameterof the Instrumented Probe Vehicle as measured by the onboard measurementsystem so as to determined the measurement accuracy of the trafficmonitoring station.